Indeed he is an ultimate badass. Flask is really great. But, especially given how beautiful the Flask docs are, I don't know what makes people think Scribd is a good idea when there are shared google docs, everyone has libre office or ms office or you can print to PDF. The last slide says he will put the slides on his personal site which has been down for a while so I guess that might be a reason.
There is no reason because it's not true. I've used the with statement to help with auto-closing tags (and just readability) in templating and it helps a lot. I've used in on sites which received over 1 million hits a month. It's been used on over a dozen sites and projects from hundreds of lines of codes and even into the tens of thousands of lines of code.
This is an example of why you should never use "new" in a title. This slide set is about Python 2.3's new features which are, at this point in time, old news. But the examples are still informative.